Methods for predicting motion of patient's tissues during treatment, such as for instance motion of a tumor or of an organ at risk (OAR), are desired in radiation therapy. Predicting motion or position of tissues of patients during treatment is notably necessary in order to avoid radiating OARs and/or in order to irradiate a tumour with a required dose.
Techniques have been developed for predicting a motion of a tissue during treatment. Four-Dimensional Computed Tomography (4D-CT) allows generating different CT images at different times, as it is known by the one skilled in the art. Time represents a fourth dimension, hence the adjective ‘Four-Dimensional’. From the different CT images, one can deduce motions of a tissue over time. Such motions result, for instance, from a patient's breathing. By knowing these displacements, a treatment plan that takes them into account can be established. It is then assumed that motions occurring during the time 4D-CT is performed also occur and are the same during actual treatment. However, 4D-CT is generally performed well before treatment, typically one to two weeks before. Hence, 4D-CT cannot accurately represents motion of tissues during treatment. For instance, it is possible that, between the time 4D-CT is performed and the time of treatment, geometry of a tumour (its position, shape, or size for instance) has changed and/or a patient's breathing cycle has varied.
Four-Dimensional Cone Beam Computed Tomography (4D-CBCT) is another technique known by the one skilled in the art. By positioning a source and a detector of a 4D-CBCT apparatus at different angular positions around an isocenter, and by recording or computing the acquisition times at which the different CT scans or CT projections are taken or acquired, one can reconstruct 3D images (named 4D-CBCT images) of a portion of a patient positioned at said isocenter for different times. Quality of 4D-CBCT images is poorer than quality of 4D-CT images. However, 4D-CBCT presents some advantages compared to 4D-CT. In particular, 4D-CBCT can be installed in a treatment room. Hence, 4D-CBCT images can be taken just before treatment and around an isocenter where a patient is positioned during treatment. That is generally not possible with a 4D-CT procedure. In order to keep precision and quality of 4D-CT images, 4D-CBCT images are generally used in order to rescale the former. For instance, if it is determined that a tumour follows a closed path during a breathing cycle from 4D-CT images, a baseline shift is determined from 4D-CBCT. This baseline shift generally comprises a space shift between closed paths observed in 4D-CT images and 4D-CBCT images, and an amplitude variation between said closed paths. From the knowledge of this baseline shift, a patient position correction can be estimated and/or a treatment plan, based on 4D-CT images can be updated.
In general, 4D-CBCT cannot be performed during treatment. Indeed, its source and detector that need to be positioned at different angular positions are generally attached to a gantry supporting a nozzle used for treatment. When delivering a radiating treatment beam, the treatment head (a nozzle for instance) cannot be positioned at the different angular positions required for the 4D-CBCT technique. Therefore, 4D-CBCT cannot be used for monitoring a patient's tissues during treatment. In other words, 4D-CBCT cannot be used for direct tissue tracking. However, it is desired to have some knowledge on tissue position during treatment, for instance to know when to deliver the treatment beam in order to correctly irradiate a tumour while at same time not irradiating OARs. If only 4D-CT and 4D-CBCT are used, assumption of same regular, periodic respiratory motion between the time 4D-CBCT is performed and the time of treatment is generally made. This is a limitation that induces a problem of precision in the determination of a position of a tissue of a patient during treatment. Therefore, it would be desirable to have a direct method of tracking a tissue of a patient during treatment.
Direct tumor tracking methods using implanted fiducial markers have the following disadvantages. Patients may face infection pneumothorax risks or from implantation. Implanted markers may easily migrate in a patient's body. Fiducial markers can interfere with a treatment beam in particle therapy. In particular, a range of a particle beam (proton beam for instance) can be modified when it is passing through the a fiducial marker. It is therefore desirable to have a tracking method not relying on implanted markers.
Techniques not using implanted markers have been reported. During treatment, radiographs are taken at different times, by fluoroscopy for instance. In order to monitor patient's tissues, these radiographs, taken during treatment, are compared with reference radiographs that are named Digitally Reconstructed Radiographs (DRRs). Each DRR generally corresponds or refers to a given time or a given temporal phase, for instance a respiratory phase. If a radiograph taken during treatment matches with a DRR, it is then determined that the position of patient's tissues corresponds to the position of patient's tissues corresponding to the given temporal phase of the selected DRR. According to another point of view, if a radiograph taken during treatment matches with a DRR, it is then determined that said radiograph is characterized by a temporal phase equivalent to the temporal phase associated with said DRR.
For generating a DRR, a CBCT or CT image corresponding to a given temporal phase (or given time) is selected from the different images generated by a 4D-CBCT or by a 4D-CT technique. Then, this CBCT or 4D-CT image is numerically projected on a planar surface. By performing this operation for different temporal phases, one can build corresponding different DRRs.
Tissue tracking or tracking of temporal phases relying on the use of DRRs has some disadvantages. First, one needs to build the different DRRs. This requires using some mathematical operations that are sometimes complex. It takes also some time, and needs computation resources. Second, the accuracy of the result from the comparison phase between a radiograph taken during treatment and a DRR is quite poor. This is mainly due to the fact that quality of a DRR is low because it results from numerical operations of projection of a 3D image on a plane. Quality is still lower if a DRR results from a projection of a CBCT image as it is generally the case. Indeed, CBCT images are often blurred because of the motion of the gantry supporting the source and the detector of a (4D)-CBCT apparatus.
Providing a method for determining a reference radiograph other than a DRR for a later use during treatment in tissue tracking or time tracking may provide advantages over these systems and methods. In particular, having a reference radiograph that would provide a result of comparison with a radiograph taken during treatment that is more precise may improve treatments.